A conventional color demodulation circuit includes a circuit that detects a burst portion in a digital chrominance carrier signal of a television signal to perform an automatic control for making an amplitude of the detected burst portion have a preset color signal amplitude level (hereinafter, this circuit will be referred to as an ACC circuit, for example a circuit as described in Japanese Published Patent Application No. Hei. 08-107549), and a circuit that detects a phase shift or an amplitude level of a burst signal that has been outputted from the ACC circuit, thereby performing chroma killer. A structure of such color demodulation circuit is schematically shown in FIG. 6.
The color demodulation circuit shown in FIG. 6 includes a band-pass filter 1, an ACC circuit 2, a multiplier 3, a phase killer detection circuit 4, an amplitude killer detection circuit 5, a chroma killer control circuit 6, and a color-difference output demodulation circuit 7.
The band-pass filter 1 extracts color components from a digital chrominance carrier signal.
The ACC circuit 2 performs an automatic control for making a signal amplitude of a burst portion in the chrominance carrier signal be at a level that is set by a color level setting signal.
The multiplier 3 multiplies an output from the band-pass filter 1 by an output from the ACC circuit 2.
The phase killer detection circuit 4 outputs a phase killer detection signal from an output of the multiplier 3 on the basis of a phase shift of the amplitude-controlled burst portion in the chrominance carrier signal.
The amplitude killer detection circuit 5 outputs an amplitude killer detection signal from an output of the ACC circuit 2 on the basis of an amplitude level of the amplitude-controlled burst portion in the chrominance carrier signal.
The chroma killer control circuit 6 outputs a chroma killer signal on the basis of the phase killer detection signal that is outputted from the phase killer detection circuit 4, and the amplitude killer detection signal that is outputted from the amplitude killer detection circuit 5.
The color-difference output demodulation circuit 7 demodulates color-difference signals (an R-Y signal and a B-Y signal) from the output of the multiplier 3 on the basis of the chroma killer signal.
Next, the operation of the conventional color demodulation circuit will be described.
The band-pass filter 1 extracts color components from an inputted chrominance carrier signal, to be multiplied by the output from the ACC circuit 2, using the multiplier 3. The ACC circuit 2 outputs a value that is adapted to the amplitude of the output from the band-pass filter 1 so as to always keep a color signal amplitude (color level) that has been previously set by the color level setting signal, while the output from the multiplier 3 is being fed back. That is, with the ACC circuit 2 and the multiplier 3, the chrominance carrier signal that is always amplitude controlled adaptively to a color level that is set by the color level setting signal is outputted to the phase killer detection circuit 4 and the color-difference output demodulation circuit 7.
The phase killer detection circuit 4 detects a phase state of the amplitude-controlled burst portion in the chrominance carrier signal, from the output of the multiplier 3, and outputs a phase killer detection signal when the burst phase is not kept at a prescribed phase state. Further, the amplitude killer detection circuit 5 detects an amplitude of the amplitude-controlled burst portion in the chrominance carrier signal, from the output of the ACC circuit 2, and outputs an amplitude killer detection signal when the burst amplitude does not reach a prescribed level.
The chroma killer control circuit 6 outputs a chroma killer signal to the color-difference output demodulation circuit 7 on the basis of the phase killer detection signal that is outputted from the phase killer detection circuit 4 and the amplitude killer detection signal that is outputted from the amplitude killer detection circuit 5.
The color-difference output demodulation circuit 7 detects a burst signal state of the inputted chrominance carrier signal on the basis of the chroma killer signal that is outputted from the chroma killer control circuit 6, and demodulates an R-Y signal and a B-Y signal from the output of the multiplier 3 on the basis of the burst signal state.
However, when instantaneous changes occur in the burst signal that is inputted to the conventional ACC circuit 2, for example when a chrominance carrier signal in which a state including a burst signal and a state without a burst signal temporally switch, as shown in FIG. 3(a), is inputted to the color demodulation circuit, it takes time to stabilize the result of an operation in the ACC circuit 2 for deciding a factor of the multiplier 3 on the basis of the state of the signal amplitude of the burst portion of the inputted chrominance carrier signal and the state of the output from the multiplier 3. Therefore, the state of the output from the ACC circuit 2 becomes unstable at a changing point between the state including a burst signal/the state without a burst signal, or in the state without a burst signal.
Further, when the chrominance carrier signal in the state where a burst signal is not included is inputted to the color demodulation circuit, the state of an output from the ACC circuit 2 becomes unstable because the operation of the ACC circuit 2 for automatically controlling the burst signal amplitude level to a preset color signal amplitude cannot be performed. Therefore, when the state of the output from the ACC circuit 2 is unstable, the R-Y signal and the B-Y signal that are outputted from the color-difference output demodulation circuit 7 are also recognized as noises on a television screen, due to abrupt variations. Further, as the output from the ACC circuit 2 is directly coupled also to the amplitude killer detection circuit 5, an erroneous detection of an amplitude killer may occur.
Further, in the conventional color demodulation circuit, the ACC circuit 2 always outputs the chrominance carrier signal of a fixed amplitude level, independently of the input amplitude level. However, when a miniscule input in such a case where the strength of the electric field of the chrominance carrier signal is weak is given to the multiplier 3, the output of the ACC circuit 2 is adversely increased by an automatic color control capability of the ACC circuit 2, and when a further miniscule input is given to the multiplier 3, the output of the ACC circuit 2 becomes saturated. As a result, the state of the output from the ACC circuit 2 becomes unstable, and the chrominance color signal cannot be kept at a fixed amplitude level. Further, when the input to the multiplier 3 becomes too small, the amplitude level is attenuated. When the phase killer detection is performed in this state by the conventional structure that performs the phase killer detection on the basis of the burst amplitude, a phase killer would be detected while the phase is proper, i.e., there occurs an erroneous detection of the phase killer.
As described above, the conventional color demodulation circuit cannot address temporal changes in the burst signal or changes in the electric field state of the chrominance color signal, whereby it is difficult to perform a stable killer detection and color-difference output.